Inside The Exam Room™ 12/18/2006
INTRAVASCULAR STENTS: Part 1
By Mark Ombrellaro, MD
Intravascular stents have received a lot of attention in the press over the last several weeks and there is not a day that goes by that several patients will ask about what the issues are and why all of the news? To answer their question, it is best to begin with an understanding of what stents actually are and how they are designed to help our circulation.
The ideal result from minimally invasive endovascular intervention is to restore the diseased blood vessel to one with a large, smooth edged, clot resistant flow channel. As you may recall from previous blogs, angioplasty is a minimally invasive technique that can be used to improve blood flow through a narrowed or blocked section of artery by using a balloon to make a controlled tear within the artery in order to stretch it out. Following angioplasty, it was found that in some circumstances there was less than an optimal result after the dilatation procedure. Some arteries with extensive or calcified plaque had residual (continued) narrowing of the artery lumen (flow channel), some had extensive tearing of the inner lining of the artery (dissection) producing a large flap that interfered with the blood flow or put the patient at risk for developing clotting within the arterial segment, and some had significant recoil of the artery which caused it to shrink right down to its original size once the angioplasty balloon was deflated. Because of these issues, there became the need to develop some type of intravascular support system that would serve to prop open the arterial lumen when these circumstances occurred. Stents are that support system: a tubular scaffolding originally designed to overcome the limitations of balloon angioplasty.
Dotter is again credited with developing the first stent. In 1964, he tried inserting a plastic tube inside several arteries to keep them propped open. He found that with these devices they migrated in the blood stream (did not stay in place), and that they all clotted off within the first day. He next tried a device constructed of stainless steel in a coil spring configuration. Some were bare metal and others were coated in silicone. While all of the silicone stents also clotted immediately, the bare metal wire stents remained open. After 2 years, the bare metal stents were found to be covered over with scar tissue and endothelium (the normal cell lining of the inside of arteries) but there had also developed narrowing within the artery lumen as part of the healing process. Dotter recognized that an open architecture (a tube with openings throughout its surface rather than a complete metal sleeve) allowed tissue to grow in and around the arms of the stent and help heal the artery, incorporate the foreign body into its wall, and keep its lumen open longer. Over the next 15 years, Dotter and others developed various types of self expanding stent prototypes with varying results. In 1985, Palmaz developed a stent that was made from continuously woven stainless steel wire that was silver soldered at the various cross points. The stent design was that of a wire-mesh tube that was loaded onto a balloon angioplasty catheter. Palmaz found that his design could be delivered accurately and implanted in a variety of vessels by changing the carrier balloon. In addition, the open strut architecture allowed for rapid ingrowth of tissue and incorporation into the arterial wall. In 1986, he reported a version of his stent design which was made from a small diameter stainless steel tube with eight rows of offset slots laser cut into the metal. This design is the basis for our current balloon expandable stents. With the Palmaz stent, as the balloon is inflated and expands the metal, the slots become stretched and the spaces assume a diamond configuration.